1. Field of the Invention
The invention relates to a variable gain amplifier circuit, and more particularly to a variable gain amplifier circuit with offset compensation.
2. Description of the Related Art
FIG. 1 shows a conventional variable gain amplifier circuit with an offset compensation. With switches SW11, SW12, and SW13, a gain for the VGA circuit can be determined according to the resistances of the resistors R11˜R14 and a bias voltage signal Vb. A current source 11 provides a current Iofs to the node N13 (referred to current injection node) between the resistors R13 and R14 for offset compensation. The DC level of the output voltage signal Vout can be controlled by adjusting the amount of the current Iofs. However, if there is a voltage difference (so-called DC offset) between an input voltage signal Vin and the bias voltage signal Vb or the amplifier 10 initially has an input offset itself, the current source 11 will be unable to provide an appropriate current Iofs for performing the correct offset compensation while the VGA circuit is set to have a high gain.
FIG. 2 is an equivalent circuit illustrating the conventional variable gain amplifier circuit with an offset compensation in FIG. 1 when the switch SW11 is turned on. Referring to FIGS. 1 and 2, the sum of the resistances of the resistors R21 and R22 of FIG. 2 is equal to the sum of the resistances of the resistors R11˜R13 of FIG. 1, and the resistor R23 of FIG. 2 corresponds to the resistor R14 of FIG. 1. Thus, the output voltage signal Vout is obtained by:
                                                        Vout              =                            ⁢                                                Vin                  (                                      1                    +                                                                                            R                          ⁢                                                                                                          ⁢                          22                                                +                                                  R                          ⁢                                                                                                          ⁢                          23                                                                                            R                        ⁢                                                                                                  ⁢                        21                                                                              )                                -                                  Vb                  (                                                                                    R                        ⁢                                                                                                  ⁢                        22                                            +                                              R                        ⁢                                                                                                  ⁢                        23                                                                                    R                      ⁢                                                                                          ⁢                      21                                                        )                                -                                  IofsR                  ⁢                                                                          ⁢                  23                                                                                                        =                            ⁢                              Vin                +                                                      (                                                                                            R                          ⁢                                                                                                          ⁢                          22                                                +                                                  R                          ⁢                                                                                                          ⁢                          23                                                                                            R                        ⁢                                                                                                  ⁢                        21                                                              )                                    ⁢                                      (                                          Vin                      -                      Vb                                        )                                                  -                                  IofsR                  ⁢                                                                          ⁢                  23                                                                                        (                  Equation          ⁢                                          ⁢          1                )            
The voltage Vn13 at the current injection node N13 is obtained by:
                                                                        Vn                ⁢                                                                  ⁢                13                            =                              Vin                +                                                      (                                          Vout                      -                      Vin                                        )                                    ⁢                                      (                                                                  R                        ⁢                                                                                                  ⁢                        22                                                                                              R                          ⁢                                                                                                          ⁢                          22                                                +                                                  R                          ⁢                                                                                                          ⁢                          23                                                                                      )                                                                                                                          =                              Vin                +                                                                            R                      ⁢                                                                                          ⁢                      22                                                              R                      ⁢                                                                                          ⁢                      21                                                        ⁢                                      (                                          Vin                      -                      Vb                                        )                                                                                                          (                  Equation          ⁢                                          ⁢          2                )            
It is assumed that there is a DC offset between the input voltage signal Vin and the bias voltage signal Vb, e.g. (Vin-Vb). According to Equation 2, as
      R    ⁢                  ⁢    22        R    ⁢                  ⁢    21  increases, the voltage Vn13 at the current injection node N13 is raised or lowered, such that a transistor within the current source 11 can not be normally operated in the saturation region when the voltage Vn13 is excessively large or small. That is, the current source 11 may not provide the appropriate current Iofs when VGA circuit has a high gain for the input voltage signal Vin, for example, when the switch SW11 or SW12 is turned on.
It is desired to provide a variable gain amplifier circuit which can be selectively applied a current at one of the current injection nodes in order to achieve offset compensation.